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- 28th March 2011, 19:21 #1

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## AC Analysis of single opamp differential amplifier Pspice simulation

Hi,

Im trying to do a ac analysis of a opamp differential amplifier the circuit is shown below.

The input is fed with a common mode ac signal 1 vpp.

The differential stage is given a 6V offset too..

So for the common mode signal the output ideally will be 6V dc signal only.But when i do the AC analysis im getting something like this..

Can somebody tell me why

- 28th March 2011, 19:21

- 28th March 2011, 23:48 #2

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## Re: AC Analysis of single opamp differential amplifier Pspice simulation

The transient in the waveform is around BW cutoff of the circuit. Can you elaborate more, what exactly are you showing? Is is written there "N differential..?"

- 29th March 2011, 00:04 #3

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## Re: AC Analysis of single opamp differential amplifier Pspice simulation

Your circuit measures common mode rejection ratio, as expectable for any real OP, it's below infinity...

The measurement correspondends almost exactly to the datasheet's CMRR specification of about 60 dB at 1 MHz.

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- 29th March 2011, 02:34 #4

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## Re: AC Analysis of single opamp differential amplifier Pspice simulation

Your circuit measures common mode rejection ratio, as expectable for any real OP, it's below infinity...

The measurement correspondends almost exactly to the datasheet's CMRR specification of about 60 dB at 1 MHz.

what im asking is that,,,,Isn't the output voltage should rise frm the offsetted 6v dc rather than from 0v dc,after 100kHz?

For zero differential input i ve offsetted the opamp at 6v dc.

Thanks For any help

RegardsLast edited by themaccabee; 29th March 2011 at 06:35.

- 29th March 2011, 02:34

- 29th March 2011, 10:51 #5

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## Re: AC Analysis of single opamp differential amplifier Pspice simulation

or does the ac analysis will kill all the dc sources?? and show only relative values?? or is there some error in my circuit?

- 29th March 2011, 11:20 #6

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## Re: AC Analysis of single opamp differential amplifier Pspice simulation

or does the ac analysis will kill all the dc sources??

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- 29th March 2011, 11:22 #7

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## Re: AC Analysis of single opamp differential amplifier Pspice simulation

Your sim results are as expected. AC analysis doesnot kill dc sources, it will take dc points before calculating AC analysis data. You are not offsetting the opamp as you mentioned but giving common mode signal on dc 6V . Are u clear?

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- 29th March 2011, 11:22

- 29th March 2011, 12:10 #8

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## Re: AC Analysis of single opamp differential amplifier Pspice simulation

Your sim results are as expected. AC analysis doesnot kill dc sources, it will take dc points before calculating AC analysis data. You are not offsetting the opamp as you mentioned but giving common mode signal on dc 6V . Are u clear?

Vo=Vref+Vdiff*G

Vref=6V

G=1

So thats becoz i asked so.

I think now it is becomming clear..

Thanks..

- 29th March 2011, 12:38 #9

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## Re: AC Analysis of single opamp differential amplifier Pspice simulation

AC analysis is small signal analysis. It determines the change in output voltage generated by a small (or as the mathematicans say

*infinitesimal*) change in input voltage. So the DC level is always ignored.

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